The H-protein is one of the four essential components (H-, L-, P-, and T-proteins) of the mammalian glycine cleavage enzyme complex, the major degradative pathway of glycine. We have isolated the full-length cDNA of the H-protein gene from the rabbit (Oryctolagus caniculus) by reverse transcription of liver poly-A mRNA and determined its nucleotide sequence (GenBank Acc. No. BankIt 318281 AF 231451). Similar to that in human, the rabbit H-protein gene possesses a 519-bp open reading frame that translates a 173-amino-acid (aa) protein. This reading frame is comprised of a 48-aa mitochondrial targeting sequence and a 125-aa residue that constitutes the mature mitochondrial matrix protein. In the mature protein region, there is a 95.5% nucleotide and 98.4% amino-acid sequence similarity to human. This conservation was also noted in the mature protein of the cow (Bos taurus) and chicken (Gallus domesticus), where there are a 94.1% and 85.3% nucleotide similarities, and 95.2% and 85.6% amino-acid sequence similarities, respectively. However, the targeting region is not as well conserved. Comparison of the rabbit targeting sequence to that in human, cow, and chicken reveals 84.0%, 79.2%, and 72.9% nucleotide, and 72.9%, 75.0%, and 54.2% amino-acid sequence similarities, respectively. These findings suggest that within the H-protein gene, the regions encoding the mitochondrial targeting and matrix protein may have evolved differently. Gene diversification in the former may reflect the species specificity in targeting proteins destined for the mitochondria, whereas homology in the latter suggests a very similar structure-function of the mature H-protein among these species. This homology in structure-function likely accounts for the observation that non-human H-protein can replace the human protein in the activity assay of the glycine cleavage enzyme system. This includes the biochemical diagnosis of non-ketotic hyperglycinemia (NKH) resulting from defects other than the H-protein, e.g., mutation(s) in the P-protein.Key words: glycine cleavage enzyme, H-protein, sequence comparison, non-ketotic hyperglycinemia.
The glycine cleavage system. The coupled expression of the glycine decarboxylase gene and the H-protein gene in the chicken.
